As heat is one of the excitant agencies essential to the support of the vital functions at their healthy standard, it follows that cold, which is simply a diminution of heat, must be attended with depression of these functions; in other words, must operate as a sedative; and, as heat is universal in its influence in one direction, so also must cold be in the other.

Though thus directly depressing, cold is in two methods indirectly excitant; first, by the shock produced on the nervous centres through the sensation it occasions; and, secondly, by the reaction which follows its primary depressing effect, through the increased excitability acquired by the repose of the function. These indirect influences of cold have been sufficiently considered, the former under the head of the nervous stimulants (i. 587), and the latter under that of the tonics (i. 194). With the depressing effects of cold are combined a contraction and an increased firmness of the tissues, which render it susceptible of various therapeutic applications, already fully treated of under the astringents (i. 108). in consequence of its directly depressing and secondarily excitant effects, cold is a prolific cause of disease. in this relation, I have treated of it at length in my work on the Practice of Medicine. Upon the points above mentioned, I must content myself with directing the attention of the reader to the several references just given. in this place, cold is to be considered exclusively in its capacity of a direct sedative.

It will be proper to say, preliminarily, that cold, in reference to impressions made upon the body, is a relative term, applicable to a variable temperature, which depends partly on the conducting power of the substance through which the impression is to be made, and partly upon the heat of the body at the time. Thus, a good conductor of heat, like iron, will feel much colder than a bad conductor, like wood; and a substance which feels cold when the skin is hot, may feel warm in an opposite state of the surface. To obtain the effects of cold, the temperature of the medium must be lower than that of the surface at the time of application. With a view to the sedative effects, water or ice is the medium usually employed; though recourse is sometimes had to the evaporation of volatile substances, as alcohol or ether. The impression is increased, when cold water is used, by maintaining a current over the part; the portion in contact with the surface being thus replaced before it becomes warm. in relation to the process of evaporation, the degree of cold can be greatly augmented by blowing over the part by means of a pair of bellows, and thus increasing the rapidity of the evaporation, and, of course, the reduction of temperature. An intense cold can thus be obtained by means of ether, applied to the surface on a single layer of linen or cotton cloth. But a still more powerful effect is obtained by the application of ether, or other volatile liquid, in the form of spray, as produced by the atomizer. in this way the temperature of the surface has been reduced to 16° below the zero of Fahrenheit.